Tuning the Photoelectrocatalytic Hydrogen Evolution of Pt-Decorated

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Tuning the Photoelectrocatalytic Hydrogen Evolution of Pt-decorated Silicon Photocathodes by Pt Electroless Deposition Temperature and Time Bruno Fabre, Gaozeng Li, Francis Gouttefangeas, Loic Joanny, and Gabriel Loget Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02122 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 26, 2016

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Tuning the Photoelectrocatalytic Hydrogen Evolution of Pt-decorated Silicon Photocathodes by Pt Electroless Deposition Temperature and Time Bruno Fabrea,*, Li Gaozenga, Francis Gouttefangeasb, Loic Joannyb and Gabriel Logeta,* a

Institut des Sciences Chimiques de Rennes (ISCR), UMR 6226 CNRS/Université de Rennes 1, Matière Condensée et Systèmes Electroactifs (MaCSE), Campus de Beaulieu, 35042 Rennes Cedex, France

b

ScanMAT – CMEBA, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France

KEYWORDS. Hydrogen evolution reaction; Hydrogen-terminated silicon; Electroless metal deposition; Photocathodes; Photoelectrocatalysis

ABSTRACT. The electroless deposition of Pt nanoparticles (NPs) on hydrogen-terminated silicon (H-Si) surfaces is studied as a function of temperature and the immersion time. It is demonstrated that isolated Pt structures can be produced at all investigated temperatures (between 22 and 75°C) for short deposition times, typically within 1-10 min if the temperature was 45°C or less than 5 min at 75°C. For longer times, dendritic metal structures 1 ACS Paragon Plus Environment

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start to grow leading ultimately to highly rough interconnected Pt networks. Upon increasing temperature from 22 to 75°C and for an immersion time of 5 min, the average size of the observed Pt NPs monotonously increases from 120 to 250 nm and their number density calculated by scanning electron microscopy decreases from (4.5 ± 1.0) × 108 to (2.0 ± 0.5) × 108 Pt NPs cm-2. The impact of both the morphology and distribution of Pt nanoparticles on the photoelectrocatalytic activity of the resulting metallized photocathodes is then analyzed. Pt deposited at 45°C for 5 min yields photocathodes with the best electrocatalytic activity for the hydrogen evolution reaction (HER). Under illumination at 33 mW cm-2, this optimized photoelectrode shows a fill factor (FF) of 45%, an efficiency (η) of 9.7% and a short-circuit current density (|

|) at 0 V vs reversible hydrogen electrode (RHE) of 15.5 mA cm-2.

1. Introduction. The conversion of sunlight into clean and storable fuels is very appealing in the frame of the actual growing need for sustainable sources of energies. To this end, the use of solar-driven electrochemical cells that can split water into hydrogen and oxygen seems a very promising technology.1,2,3 Nevertheless, the fabrication of stable, low-cost and really efficient photoelectrodes is still a harsh challenge in this thematic area.4,5,6,7 Silicon, with a gap energy of 1.12 eV,8 is a very promising material to be used as a photocathode material for hydrogen evolution reaction (HER) because of its abundance, non-toxicity and its tunable electronic properties.9 However, slow charge transfer kinetics at bare Si and the high recombination rate of electron-hole pairs generated under illumination constitute serious obstacles to promote efficiently multielectron processes.9,10 Therefore, the deposition of pure metals9, metal oxides or sulfides4,6,11,12 onto silicon is an effective way to overcome the intrinsic problems of this semiconductor serving as a photocathode. Among the catalytically active metals, platinum is known as the most efficient electrocatalyst for HER which can 2 ACS Paragon Plus Environment

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remain stable either in acidic or alkaline electrolyte.9,13,14,15,16,17,18,19,20,21,22,23 It has been demonstrated that Pt, deposited in the form of nanoparticles (NPs), exhibited much higher electrocatalytic activity than that of a continuous Pt film.9,24 Furthermore, the catalytic performances of Pt NPs were found to be highly dependent on their size, their morphology and their shape.25,26,27,28,29,30 In that context, attention has been paid to the development of synthetic procedures directed towards the deposition of Pt NPs with high-index facets.31,32,33,34 Indeed, the high-index facets are usually more active in chemical reactions and heterogeneous catalytic processes compared with the low-index planes of closely packed surface atoms. As a matter of fact, a surface with high-index planes possesses a high density of terraced, stepped and kinked sites which serve as active sites for a range of reactions. A common route for the synthesis of such shape-controlled Pt NPs involves the gas-phase reduction of a precursor Pt salt in the presence of a capping agent.26,27,35,36,37 Electrochemical methods such as potential cycling38 and square-wave potential voltammetry31,39,40,41 have also been successfully employed for the deposition of Pt NPs with high-index facets onto glassy carbon and graphite surfaces. In the present work, we exploit another electrochemical approach, namely the electroless deposition by galvanic displacement42,43, to grow Pt NPs with different shapes onto photoactive silicon surfaces. In the past, this surfactant-free method has been demonstrated to be a simple and fast procedure to yield Pt NPs-decorated semiconducting surfaces exhibiting improved interfacial charge transfer rates and hydrogen evolution performances.14,17,18,19,22,23 We demonstrate herein that both the morphology and the density of Pt nanoparticles deposited on silicon can be finely tuned by electroless deposition temperature and time. In our study, a wide range of temperature (20-75°C) and time (1-30 min) has been used to produce a large palette of structurally different Pt nanostructures going from separated spherical nanoparticles to interconnected dendritic structures. In contrast, most of reported electroless metal deposition methods were conducted at ca. room temperature for a few minutes and a single 3 ACS Paragon Plus Environment

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type of metal morphology was usually deposited. To the best of our knowledge, this is the first time that the impact of both deposition temperature and time on the electrocatalytic performance of the resulting metallized silicon photocathodes for HER has been addressed with the goal of identifying the most efficient photocathode, in terms of short-circuit current density (Jsc), fill factor (FF) and thermodynamic efficiency for production of H2 (η).5

2. Experimental 2.1. Reagents. Acetone (MOS electronic grade, Erbatron from Carlo Erba) and anhydrous ethanol (RSE electronic grade, Erbatron from Carlo Erba) were used without further purification. The chemicals used for the cleaning and etching of silicon wafer pieces (30% H2O2, 96-97% H2SO4 and 50% HF solutions) were of VLSI (H2O2, from Sigma-Aldrich) and MOS (H2SO4 from O-BASF and HF from Sigma-Aldrich) semiconductor grade. 2.2. Preparation of Pt-decorated silicon surfaces. A piece of p-type Si(100) (1-5 Ω cm resistivity, boron doped, 250 µm thickness, Siltronix) was sonicated for 10 minutes successively in acetone, ethanol and ultra-pure 18.2 MΩ cm water (Veolia Water STI) to degrease it. It was then cleaned with piranha solution (3:1 v/v concentrated H2SO4/30% H2O2) for 30 min at 100ºC, followed by copious rinsing with ultra-pure water to eliminate heavy metals and organic residues. Caution: The concentrated aqueous H2SO4/H2O2 (piranha) solution is very dangerous, particularly in contact with organic materials, and should be handled extremely carefully. Hydrogen-terminated Si(100) (H-Si) was obtained by immersing the Si(100) substrate previously decontaminated with piranha solution into HF 10% wt (∼5.7 M) for 2 min, followed by drying under an argon stream. Caution: Proper precautions must be used when handling hydrogen fluoride. Hydrogen fluoride is extremely corrosive to human tissue, contact resulting in painful, slow-healing 4 ACS Paragon Plus Environment

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burns. Laboratory work with HF should be conducted only in an efficient hood, with the operator wearing a full-face shield and protective clothing. Pt-coated silicon surfaces were obtained by electroless deposition in the presence of HF. A freshly prepared H-Si sample was dipped for a time varying between 1 and 30 min into a solution of Na2PtCl4 (Strem Chemicals) at 1 mM in HF 10%. The metallic salt solution was previously kept at rt or heated at 45°C, 60°C or 75°C for 30 min before the introduction of HSi and kept at the selected temperature throughout the course of the reaction. The metallized surface was thoroughly rinsed with ultrapure water and dried under an argon stream. It was then immediately used for (photo)electrochemical measurements. 2.3. Electrochemical Characterization. All the electrochemical experiments were performed in a home-made three-electrode glass cell comprising a quartz window. The electrical contact with the working electrode was made by applying a drop of In-Ga eutectic (Alfa Aesar, 99.99%) on the previously polished backside of the silicon electrode. KCl saturated calomel electrode (SCE) was used as the reference electrode. A glassy carbon rod was used as the counter electrode. Potentials vs SCE were converted into potentials vs reversible hydrogen electrode (RHE) from measurements of the open-circuit potential (OCP) of a clean Pt electrode in the pH 1 solution saturated with research grade H2 (Air-Liquide, 99.999%). The OCP potential was measured to be -0.300 ± 0.005 V vs SCE, in close agreement with the theoretically predicted value by the expression: VRHE = VSCE + 0.24 + 0.059*pH. All the linear sweep voltammograms were iR-corrected using the resistance determined by impedance spectroscopy at high frequency (100 kHz). Typical values of 26 ± 5 ohm were obtained for our different photoelectrochemical cells and were found to be not really dependent on the type of metallized photoelectrode. Electrochemical Measurements. Linear sweep voltammetry (LSV) and chronoamperometry were

performed

with

an

Autolab

electrochemical

analyzer

(PGSTAT

30

potentiostat/galvanostat from Eco Chemie B.V.) equipped with the GPES and FRA software. 5 ACS Paragon Plus Environment

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The aqueous solution containing sodium sulfate (Na2SO4, Carlo Erba, 99.6%) at 0.5 M was adjusted to pH 1 with H2SO4 and was continuously stirred with a stir bar during the electrochemical measurements. We have checked that the addition of Na2SO4 into the acidic solution did affect neither the potential scale nor the photoelectrochemical measurements at metallized H-Si photocathodes. The geometrical area of silicon exposed to light was 0.2 cm2. All the electrochemical measurements were carried out inside a home-made Faraday cage at room temperature and under a constant flux of argon. 2.4. Additional Instrumentation. Light source. The light source used for the photoelectrochemical experiments with silicon electrodes was an optical fiber (Fiber Light DC950H from Dolan Jenner), equipped with a 150 W quartz halogen. The fluence of the lamp at the position occupied by the silicon working electrode was measured as 33 mW cm-2 by using an optical power/energy meter (Model 842-PE, Newport) coupled to a silicon photodiode (Model 918D-SL-OD3, Newport). This value corresponds to the maximum power reached by our light source. A Xe lamp allowing illumination at 100 mW cm-2, as traditionally used in this kind of photoelectrocatalytic experiments, was not currently available in our lab. Scanning Electron Microscopy. A JEOL JSM-7100F Scanning Electron Microscope (SEM) was used to analyze the morphology of Pt-coated silicon surfaces as a function of both the metal deposition temperature and time. The images were processed by the ImageJ (version 1.45s) software to determine the number density of Pt particles deposited on H-Si(100) surfaces. The procedure to count the Pt particles was as follows. First, the "Analyze/Set Scale" dialog box was used to define the spatial scale from the scale bar in the SEM images. The "Analyze particles" automated routine was then used to count and outline the Pt particles. The data window was copied and pasted to a OriginPro 8.5 spreadsheet.

3. Results and Discussion.

6 ACS Paragon Plus Environment

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3.1. Morphology of the Pt NPs deposited on H-Si. Figure 1 shows the SEM images of Pt NPs deposited on H-Si by electroless galvanic displacement at different temperatures for 5 min from an aqueous HF-containing metallic solution at 1 mM. The Pt NPs were of hexoctahedron shape44 when the deposition temperature was fixed between 22 and 60°C. For a temperature of 75°C, a mixture of hexaoctahedra and highly branched structures was observed. Furthermore, the density of deposited Pt NPs decreased upon increasing the deposition temperature from 22 to 75°C. The number density calculated by SEM (Figure S1, Supporting Information) was around (4.5 ± 1.0) × 108, (3.5 ± 0.8) × 108, (3.0 ± 0.6) × 108 and (2.0 ± 0.5) × 108 Pt NPs per cm2 of the silicon surface at 22, 45, 60 and 75°C, respectively. Moreover, the average size of the observed features monotonously increased from 120 to 250 nm within this temperature range with an approximately constant size distribution until 60°C enlarging at 75°C (the width at half-maximum was found to be 35 ± 5 nm within the 22-60°C range and increased to 120 ± 10 nm at 75°C). It is worth mentioning that the average surface occupied by the Pt particles was determined to be around 9 ± 1% of the total silicon surface and was found to be not significantly dependent on the deposition temperature. For all investigated temperatures, the metal deposition mechanism involved the attack of the bulk silicon concomitantly with the growth of isolated Pt NPs which resulted ultimately in the formation of nanotrenches surrounding the Pt NPs (Figures 1 and S2). The morphology of the grown Pt NPs was not only dependent on the deposition temperature but also on the immersion time. The latter parameter has been examined more in detail for two representative temperatures: 45 and 75°C. For 45°C, Pt NPs were essentially of hexoctahedron shape when the deposition time did not exceed 10 min. For times longer than 15 min, highly branched structures, grown from the corners of the polyhedral, were observed leading to the formation of a few hundred nm long Pt dendrites after 30 min deposition (Figure 2). When the temperature was fixed at 75°C, the transition between hexoctahedron and highly branched


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structures occurred at shorter times, between 1 and 5 min. Long dendrites standing out of silicon trenches were clearly visible from 10 min (Figure 3). To sum-up, isolated Pt structures can be produced at all investigated temperatures for short deposition times, typically within 1-10 min if the temperature was 45°C or less than 5 min at 75°C. For longer times, dendritic metal structures start to grow leading ultimately to highly rough interconnected Pt networks. As detailed in the next section, such differences in both the morphology and the density of deposited Pt NPs have a strong impact on the electrocatalytic performances for HER of metallized surfaces.


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22°C

45°C

60°C

75°C

1 µm

200 nm


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Figure 1. (Top, middle) SEM Images of flat H-Si(100) surfaces after electroless deposition of Pt at different temperatures for 5 min. Deposition solution: Na2PtCl4 at 1 mM in HF 5.7 M. (Bottom) Corresponding particle size distributions.

1 min

5 min

10 min

15 min

30 min

1 µm

200 nm Figure 2. SEM Images of flat H-Si(100) surfaces after electroless deposition of Pt at 45°C for 1, 5, 10, 15 and 30 min. Deposition solution: Na2PtCl4 at 1 mM in HF 5.7 M.


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1 min

5 min

10 min

15 min

30 min

1 µm

200 nm Figure 3. SEM Images of flat H-Si(100) surfaces after electroless deposition of Pt at 75°C for 1, 5, 10, 15 and 30 min. Deposition solution: Na2PtCl4 at 1 mM in HF 5.7 M.


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3.2. Electrocatalytic HER at platinized H-Si photocathodes. Figure 4 shows the iR-corrected polarization curves obtained at Si photocathodes which had been previously metallized at different temperatures for 5 min. As expected, no appreciable reduction current was measured in the dark for all the electrodes. Under illumination with a halogen lamp (33 mW cm-2), they exhibited much higher electrocatalytic performance than that of nonmetallized H-Si(100). For example, the overpotential for hydrogen evolution was significantly decreased by ca. 940 mV with respect to H-Si(100) (measured for a current density of 5.0 mA cm-2) under identical illumination conditions. We now examine in more details the effects of the metal deposition temperature on the electrocatalytic behavior of the photoelectrodes. Qualitatively, the cathodic photocurrent densities corresponding to HER were significantly higher at photocathodes metallized at temperatures above 45°C. In contrast, the open-circuit voltage Voc (the difference between the potential at which the photoelectrode passed no current and the formal potential of the H+/H2 redox couple)5 was constant at 450 ± 10 mV vs RHE and not dependent on the deposition temperature. For all the metallized photoelectrodes, it is worth noting that the catalytic photocurrent decreased slightly for potentials below 0.0 V vs RHE instead of being flat as expected for light-limited catalytic current. Such a trend is thought to be due to the gradual decrease in the active surface area of the photoelectrode caused by the accumulation of H2 bubbles which were not totally removed from the surface upon stirring. To evaluate quantitatively the respective catalytic performance of each metallized electrode, two other characteristic parameters have been determined, namely the fill factor (FF, eq 1) and the thermodynamic efficiency for H2 evolution (η, eq 2).5

=

∙|

|

(1)


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=

max

=

∙

∙|

|

(2)

in

where Pmax is the maximum product of | | and E, |

| is the short-circuit current density (at E =

0 V vs RHE) and Pin is the incoming light intensity (i.e. 33 mW cm-2). The results are gathered in Figure 5. The values of FF (25%), η (90%. O2 evolution at the Pt counter electrode was also monitored and the overall molar ratio of the evolved H2:O2 was 2.5 ± 0.2 (Figure 9b). These deviations from the expected Faradaic yield of 100 % and stoichiometric ratio of 2:1 are believed to be due to the uncertainty in the gas volume measurements with the Hoffman cell and to a possible reduction of a small fraction of the O2


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generated at the anode. However, these results clearly show that such photocathode is robust enough to be operated for H2 production during a reasonable amount of time.

Figure 9. (a) Current-time curve obtained by controlled-potential electrolysis at 0.0 V vs RHE for the photoelectrode prepared at 45°C for 5 min in H2SO4 solution containing 0.5 M Na2SO4 (pH 1) under illumination at 33 mW cm-2 (ON state) with two dark periods (OFF state). (b) Amount of H2 and O2 evolved at the photoelectrode and the Pt counter electrode, respectively, as a function of the electrolysis time. The number of mole of produced gas was calculated from gas volume in a Hoffman cell.

3.4. Comparison with other Pt NPs-decorated H-Si photocathodes. Table 1 lists the reported HER performances of several Pt-metallized Si photocathodes using different deposition methods (electroless, electrochemical, evaporation and atomic layer deposition ALD). Two types of electrode structures have been considered, namely planar and micro-/nanostructured silicon. Globally, the optimized photoelectrode prepared in this study exhibits the best catalytic activity, in terms of Voc, FF and η, compared with other photoelectrodes based on planar silicon. The smaller value of Jsc found herein can be reasonably ascribed to the lower illumination power.


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From Table 1, it is also evident that the electrochemical deposition methods failed to produce photocathodes with high Voc and η. Moreover, it is worth mentioning that the exceptional efficiency reported by Lombardi et al. (>10%)15 has been calculated for a three-electrode electrochemical cell using the expression available for a complete water splitting photoelectrode device5 and consequently is an overestimate of the real efficiency of the individual HER photocathode. Compared with structured photoelectrodes, our optimized planar photocathode exhibits a comparable or higher catalytic performance. The use of Si micro- and nanowires as electrode materials was found to significantly improve Voc relative to the planar electrodes. Nevertheless, the measured photocurrent densities were not significantly enhanced as expected for such high surface area structures. This well-known trend has been attributed to a less efficient light absorption by the wires essentially due to the high density of surface states.9 Furthermore, the use of a high-cost metal deposition technique, such as atomic layer deposition (ALD), yielded photocathodes with small Jsc and η, in spite of a uniform distribution of Pt NPs along the wires.20 As a matter of fact, the only configuration allowing outstanding HER performance included a n+ p radial junction in planar or Si wire-array structures.18 FF higher than 50% with Voc of ca. 0.55 V vs RHE were measured for such photoelectrodes.


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Table 1. Key HER catalytic parameters for different Pt metallized silicon photocathodes. Pt

Pt deposition

Illumination

deposition

conditions

power

methoda

pH

Voc / V vs

-Jsc / mA

FF

η

RHE

cm-2

/%

/%

Ref.

/ mW cm-2 Planar p-Si

EL

5 min@45°C

33

1

0.46

15.5

45

9.7

This work

EL

3-8 min@rt

100

4.5

0.21-0.25

10-22

12-22

0.3-1.1

17

Echem

-2 Vb for 4.5 s

100

4.5

0.27

21.5

33

1.9

16

EL

2-10 min@rt

10-100

1

∼0.30

2-20c

d

25-10c,e

15

EL

∼10 min@rt

100

2

0.30

23

30

2.1

18

Echem

-0.3 Vf

6.4-11.8g

1.3

0.26

0.8h

d

2.7-3.5

13

Micro-/Nanostructured p-Si EL

∼10 min@rt

100

2

0.16

7.3

18

21

18

EL

1 min@rt

100

0

∼0.40

28

d

d

19

EL

3 × 3 min@rt

100

1

0.28

23

28

1.8

20

ALD

50 cycles

100

1

0.31

9

55

1.5

20

@250°C EL

3 min@rt

100

1

0.42

18

d

d

22

EL

10-20 min@rt

100

1

∼0.40

20-25

d

d

23

28

60

∼10.0

18

15

71

∼6.0

18

Planar n+ p-Si ∼ 6 × 10-6 torr

EBE

100

2

0.56

pressure

Microwires n+ p-Si ∼ 6 × 10-6 torr

EBE

100

2

0.54

pressure a

EBE: electron-beam evaporation; EL: electroless; Echem: electrochemistry; ALD: atomic layer deposition.

Ag/AgCl.

c

-2

Measured at different illumination powers from 10 to 100 mW cm .

d

Not indicated.

e

b

Vs

Conversion

efficiency for a complete water splitting photoelectrode device. Vs SCE; Pt was deposited by passing ∼30 mC cm-2 f


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of cathodic charge.

g

632.8-nm light.

h

Calculated from photocurrent values and the 3-mm diameter of the

photoelectrode.

4. Conclusions. In this work, Pt-metallized silicon photoelectrodes, highly active for HER were fabricated by using the cost effective electroless deposition method by galvanic displacement. It was demonstrated that the morphology, the particle size and the surface distribution could be finely tuned by controlling the metal deposition temperature and time. Isolated Pt structures were produced at all investigated temperatures for short deposition times, typically within 1-10 min if the temperature was 45°C or less than 5 min at 75°C. The formation of multifaceted isolated Pt islands and nanotrenches into the silicon bulk were found to be beneficial for improving the catalytic performance of the photoelectrodes for HER. In contrast, the HER performance strongly decreased for photoelectrodes incorporating a high Pt loading with interconnected dendritic structures. The best catalytic performance was obtained for a photoelectrode which had been previously metallized at 45°C for 5 min. Such a photoelectrode was highly active for HER at pH 1 with outstanding output characteristics (FF = 45%, η = 9.7% and |

| = 15.5 mA cm-2) under a light illumination of 33 mW cm-2. Such performances could be

even improved by using Si structured surfaces incorporating an additional n+ Si layer on the ptype Si surface of the photocathode structure. Work directed towards this stimulating prospect is currently underway in our laboratory.

Supporting Information. Additional SEM images and total reflectance spectra of Pt-coated photoelectrodes. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (B.F.). Tel: +33 (0) 223 236 550. Fax: +33 (0) 223 236 797. * E-mail: [email protected] (G. L.) Author Contributions B. Fabre and G. Loget designed research, analyzed the electrochemical data and wrote the paper. L. Gaozeng performed research. F. Gouttefangeas and L. Joanny performed the SEM characterizations and analyzed the SEM data. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge CNRS and the Université de Rennes for support. F. Gouttefangeas and L. Joanny are also grateful to the Région Bretagne and European Union (CPER-FEDER 2007-2014) for financial support for SEM characterizations.


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Table of Contents Graphic

5 min @ 45°C

1 µm


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REFERENCES

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